Dual-sided membrane sheets comprise a planar configuration including two opposing outer surfaces surrounding an inner fluid flow pathway. One known embodiment includes an inner layer of a fibrous support material comprising a web or scrim of non-woven or woven fabric. These type of sheets are commonly referred to as “supported” membranes. Examples are described in U.S. Pat. No. 5,275,725; U.S. Pat. No. 7,048,855; US 2008/0164208; US 2008/0257817; US 2009/0071896 and US 2009/0078644. The inner layer of fibrous material is relatively strong and provides the sheet with support during membrane formation and may further serve as an inner fluid flow pathway. Processes for making continuous, supported, dual-sided membranes are described in US 2007/0286949, U.S. Pat. No. 6,776,940, and U.S. Pat. No. 6,090,441. In a typical embodiment a scrim of fibrous sheet material is pulled from a roll along a production pathway through a series of coating steps wherein layers of semi-permeable membrane are formed within or upon the scrim. Once formed the resulting membrane sheet may be wound up into a roll or otherwise advanced by way of rollers that engage the outer surface(s) of the membrane sheet.
Another type of dual-sided membrane sheet comprises an integral extruded porous sheet including capillary channels which serve as an inner fluid flow pathway. This type of sheet is distinguishable from those previously described in that the sheet is “unsupported.” That is, the membrane does not include a fibrous support material. Examples are described in U.S. Pat. No. 5,046,936; U.S. Pat. No. 5,089,187; U.S. Pat. No. 5,171,493 and US 2009/0011182. Such membrane sheets may be extruded from a variety of polymer mixtures, e.g. polyether sulfone, polysulfone and polyvinylidene fluoride. U.S. Pat. No. 6,787,216; FR 2616812; JP 59-082906 and JP 11-90192 describe the use of such membrane sheets within filtration modules.
U.S. Pat. No. 6,994,789 describes another embodiment of an unsupported multilayer membrane wherein multiple layers of a polymer mixture are coated upon a continuously moving non-porous release surface, e.g. a moving belt or rotating drum. After conducing a phase inversion, the resulting multilayer membrane sheet is removed from the non-porous release surface.